This invention relates to semiautomatic defibrillators, in which an advisory algorithm advises an operator as to whether a shock should be delivered, and it is left to the operator to initiate delivery of the shock.
Semiautomatic defibrillators are well known. They have been in use in one form or another for nearly twenty years. An advisory algorithm analyzes a patient's electrocardiogram (ECG), and gives the operator an advisory indication of whether a shock should be delivered. Typically, the advisory algorithm analyzes the ECG for both ventricular fibrillation and shockable high rate tachycardia. If either is found, the unit will advise the operator that a shock should be delivered. The operator then simply presses a button, and the unit immediately delivers the shock. Because these units are typically used by emergency medical technicians (EMTs) with relatively little, if any, training in cardiology no indication is provided to the operator of whether the shock advisory is based on finding ventricular fibrillation or shockable tachycardia. The operator is simply advised to deliver a shock.
In manual defibrillators, the type of units used by physicians and nurses, and by highly-skilled EMTs, there is no automated analysis of the ECG by an advisory algorithm, and instead the operator makes his or her own decision whether to apply a shock based on a display of the ECG. In addition, it is typical to allow the operator to select between delivering an immediate shock, in which energy is delivered as soon as the firing buttons are depressed, or a synchronized shock, in which energy is not delivered until an R-wave has been detected. Synchronized shocks are typically used when the operator recognizes a shockable tachycardia. By synchronizing the shock, the user avoids delivery of the shock in the interval following contraction, in which the heart muscle is repolarizing and is vulnerable to being thrown into fibrillation.
Recent cardiology research has suggested that synchronized shocks may also be of benefit in treating ventricular fibrillation. Hsia et al., "Genesis of Sigmoidal Dose-Response Curve During Defibrillation by Random Shock: A Theoretical Model Based on Experimental Evidence for a Vulnerable Window During Ventricular Fibrillation," PACE, Vol. 13, pp. 1326-42 (Oct. 1990). In addition, there is other research to indicate that accurate synchronization of the shock to ventricular tachycardia improves efficacy (e.g., Li, HG, "The effects of a different shock timing during ventricular activation on the efficacy and safety of internal cardioversion for ventricular tachycardia," cited at pp. 337-8, Defibrillation of the Heart). This research suggests that there is a greater likelihood of successful defibrillation if the shock is delivered at a time when the absolute magnitude of the VF waveform is high or at specific points during ventricular activation with ventricular tachycardias.
Synchronized shocks are delivered in manual defibrillators by having the operator hold down the shock buttons until an R-wave is detected by the unit's circuitry. This can, in some instances, mean that the operator must know to keep the buttons depressed for as long as four seconds. And, because no shock is delivered if an R-wave is not detected, the operator must be trained to appreciate that, under such circumstances, a properly functioning defibrillator may not deliver a shock.
Automatic defibrillators represent the third general category of such devices. Such automatic units are typically the implanted type, which function without intervention by the patient or operator. In such units, the stimulus delivered is entirely determined by an algorithm, which on detecting a shockable tachycardia will ordinarily first attempt to use overdrive pacing to treat the condition, and only upon that therapy failing will move to a synchronized shock.